It is essentially one channel of a sampling unit and a timing unit from a 661. It has its own trigger and timebase.

The oscilloscope's timebase is usually set to a relatively slow sweep rate when using a 1S1. Alternatively, the sweep on the oscilloscope can be free-run,
and the horizontal voltage from the oscilloscope can be used to control the 1S1, as described below.

The 1S1 has a 50 Ω GR-874 connector for signal input. The plug-in supplies power (+100 V and -12.6 V) to accessories like the P6032 Cathode Follower Probe, the 281 TDR Pulser, and the 282 Probe Adapter.

The 1S1 can operate in normal self-swept mode or an external sweep signal can be applied to the 1S1.
With the 1S1's internal sweep disabled, the horizontal-in and vertical-out connections can be used
so the the 1S1 acts as a lookup table, a mapping of x to y, a function.
The 661 also has this capability, in its "A vert/B horiz" mode, which is like X-Y mode for a sampler.
In this mode, the horizontal-in voltage controls the time after the trigger event when the sample should be taken,
and the vertical-out voltage corresponds to the voltage measured at that instant.
This allows a waveform to be digitized using an arbitrarily slow DAC to generate the horizontal voltage
and ADC to read the sampled output. But perhaps more importantly, by setting a constant horizontal-in voltage,
it allows the output signal at one equivalent time instant to be processed in the time domain.
For example, this allows the voltage to be low-pass filtered, so that it can be more accurately measured.
The other reason why one might want a signal that consists of multiple sequential measurements
of the same equivalent-time instant is that statistics can be calculated on these observations.
For example, one might want to know, when testing a logic gate, what is the propagation delay
such that 99.9% of the transitions happen faster than this.

The horizontal-in voltage can also be produced, in "manual" mode, by setting a knob on the 1S1.

The sampler in the 1S1 is quite different from the sampler in the 1S2. The 1S1 uses a four-diode sampling bridge, terminated within the 1S1. The 1S2 uses a two-diode feed-through sampler, typical for TDR instruments.